Hollow, side by side type polyethylene/polypropylene conjugated fiber with high stretchability and lightweight and method for producing non-woven fabrics by use of it

ABSTRACT

Disclosed is a hollow and side-by-side type conjugated fiber which is advantageous in terms of light weight, high bulkiness and high stretchability, comprising polyethylene with a melting point of 100-140° C., a density of 0.9500-0.9700 g/cm 3 , a molecular weight distribution index of 3.0-8.0 and a melt flow index of 10-25, and polypropylene with a melting point of 150-170° C., a density of 0.9000-0.9200 g/cm 3 , a molecular weight distribution index of 3.0-8.0 and a melt flow index of 10-25. A non-woven fabric prepared using the hollow and side-by-side type polyethylene/polypropylene conjugated fiber is also disclosed.

TECHNICAL FIELD

The present invention pertains, in general, to hollow and side-by-side type polyethylene/polypropylene conjugated fibers having high stretchability, high bulkiness and light weight, and methods of preparing non-woven fabrics using the same. More specifically, the present invention is directed to a hollow and side-by-side type polyethylene/polypropylene conjugated fiber comprising polyethylene with a melting point of 100-140° C., a density of 0.9500-0.9700 g/cm³, a molecular weight distribution index (Q) of 3.0-8.0 and a melt flow index (MFI) of 10-25, and polypropylene with a melting point of 150-170° C., a density of 0.9000-0.9200 g/cm³, a Q of 3.0-8.0 and an MFI of 10-25, and a method of preparing a non-woven fabric with excellent stretchability, high bulkiness and light weight using the conjugated fiber.

PRIOR ART

Generally, polyethylene and polypropylene, which are advantageous in terms of fast crystallization rate, excellent formability and chemical resistance, are used in various applications, such as fibers or films, and in particular are suitable for use in preparation of diapers, masks, sanitary and medical non-woven fabrics due to light weight, heat insulating property and softness thereof.

There are known a number of techniques for the preparation of a non-woven fabric by use of a conjugated fiber comprising such polyethylene and polypropylene.

In this regard, in U.S. Pat. No. 5798305 and Japanese Patent Laid-open Publication No. Hei. 8-74128, there is disclosed a non-woven fabric and conjugated fibers affording the non-woven fabric, the above conjugated fibers being composed of a conjugated fiber of a side-by-side type or a sheath-core type comprising a polyethylene sheath having 0 to 1.5 methyl groups per 1000 carbon atoms in the backbone chain, a density of 0.95 to 0.965 g/cm3 and a value of Q (weight average molecular weight/number average molecular weight) of 4.5 or less, and a polypropylene core. Further, Japanese Patent Laid-open Publication No. Hei. 5-9809 discloses a conjugated fiber comprising high crystalline polypropylene having a melting point of 160° C. or higher and 7<Q<11 as a sheath, and polyethylene having the melting point of 70-145° C. or lower as a core, and a non-woven fabric using the same. Furthermore, Japanese Patent Laid-open Publication No. Hei. 5-186955 discloses a non-woven fabric having high strength and excellent softness, comprising polyethylene with density of 0.945-0.965 g/cm³ as a sheath and polypropylene as a core.

Thusly prepared polyethylene/polypropylene conjugated fibers exhibit softness and stretchability, but are decreased in heat resistance and shape stability is poor due to drastically lowered properties under high temperature environments.

DISCLOSURE OF THE INVENTION

Accordingly, it is an object of the present invention to alleviate the problems encountered in the prior art and to provide a hollow and side-by-side type polyethylene/polypropylene conjugated fiber having advantages of light weight, bulkiness and stretchability.

It is another object of the present invention to provide a method of preparing a non-woven fabric using the polyethylene/polypropylene conjugated fiber.

In accordance with an embodiment of the present invention, there is provided a hollow and side-by-side type conjugated fiber comprising as a primary component polyethylene with a melting point of 100-140° C., a density of 0.9500-0.9700 g/cm³, a value of Q (molecular weight distribution index) of 3.0-8.0 and a melt flow index (MFI) of 10-25, and, as a secondary component, polypropylene with a melting point of 150-170° C., a density of 0.9000-0.9200 g/cm³, a Q of 3.0-8.0 and an MFI of 10-25.

In accordance with another embodiment of the present invention, there is provided a method of preparing a non-woven fabric using the hollow and side-by-side type conjugated fiber.

BEST MODES FOR CARRYING OUT THE INVENTION

The present invention is characterized by using polyethylene as a primary component and polypropylene as a secondary component in a hollow and side-by-side type polyethylene/polypropylene conjugated fiber.

In the conjugated fiber, the polyethylene component has a melting point of 100-140° C., a density of 0.9500-0.9700 g/cm³, a value of Q (a ratio of weight average molecular weight to number average molecular weight) of 3.0-8.0 and an MFI (melt flow index) of 10-25.

Concerning the physical properties of the polyethylene component mentioned above, when a melting point is lower than 100° C., polyethylene is excessively hot-melt-adhered upon preparation of the non-woven fabric, which is liable to cause the fabric to be hard. Meanwhile, when a melting point exceeds 140° C., hot melt adhesiveness is decreased upon preparation of the non-woven fabric, thus lowering breaking strength. In addition, a density less than 0.9500 g/cm³ results in decreased crystallinity of polyethylene and lowered yarn strength, whereas a density exceeding 0.9700 g/cm³ results in the melting point exceeding 140° C. due to high crystallinity thereof. Further, if a Q is less than 3.0, spinnability becomes poor. On the other hand, if a Q exceeds 8.0, physical properties of the yarn are decreased owing to low molecular weight of polyethylene. Also, if an MFI is less than 10, melt viscosity is drastically increased upon conducting a spinning process, and thus die swelling is caused by the Barus effect under excessively high shear stress and tensile stress, thereby obtaining a single yarn. On the other hand, if an MFI exceeds 25, die swelling is not generated but a dripping phenomenon may occur upon performing a spinning process due to too low melt viscosity.

Additionally, as the secondary component of the conjugated fiber, polypropylene has a melting point of 150-170° C., a density of 0.9000-0.9200 g/cm³, a Q of 3.0-8.0 and an MFI of 10-25.

Regarding the physical properties of the polypropylene component mentioned above, if a melting point is lower than 150° C., the prepared non-woven fabrics are too soft to have high shape stability due to low crystallinity of polypropylene. On the other hand, if a melting point exceeds 170° C., the prepared non-woven fabrics are hardened due to high crystallinity thereof, thus not obtaining a non-woven fabric desirous in the present invention. Further, a density less than 0.9000 g/cm3 results in decreased yarn strength due to reduced crystallinity of polypropylene, whereas a density exceeding 0.9200 g/cm³ leads to the melting point exceeding 170° C. due to too high crystallinity. In addition, a Q smaller than 3.0 results in poor spinnability, while a Q exceeding 8.0 results in decreased physical properties of the yarn due to low molecular weight of polypropylene. Also, if an MFI is less than 10, a melt viscosity becomes excessively high upon a spinning process, and die swelling is caused by the Barus effect under extremely high shear stress and tensile stress, thereby obtaining a single yarn. Meanwhile, if an MFI exceeds 25, die swelling is not generated but the melt viscosity becomes lower, thus causing a dripping phenomenon upon performing a spinning process.

Therefore, the above polyethylene and polypropylene as the primary and secondary components, respectively, constitute the conjugated fiber of the present invention. As such, it is preferred that the conjugated fiber comprises a hollow and side-by-side type.

In the conjugated fiber, a component ratio of polyethylene and polypropylene ranges from 3:7 to 7:3 based on weight. Further, a hollow ratio of the hollow and side-by-side type conjugated fiber is 20-30%. Such separate components are joined along an axially extending interface, in which the interface of the polyethylene portion of the conjugated fiber and the polypropylene portion of the fiber is preferably positioned at the center of the fiber by controlling the MFI of each of polyethylene and polypropylene.

The reason why the component ratio of polyethylene and polypropylene is 3:7-7:3 is that a weight ratio of polyethylene less than 3 results in relatively high weight ratio of polypropylene, and thus the interface therebetween is eccentric toward the polyethylene portion while soft feeling and stretchability may not be exhibited due to the hollow ratio decreased to 10%. Further, if the weight ratio of polyethylene exceeds 7, the interface between polyethylene and polypropylene is eccentric toward the polypropylene portion, attributed to relatively high weight ratio of polyethylene. Additionally, the hollow ratio is decreased to 10%, thus not exhibiting stretchability as well as decreasing shape stability due to excessive softness.

With the intention of reducing weight and increasing shape stability of the fiber, it is preferred that the hollow ratio of the hollow and side-by-side type conjugated fiber ranges from 20 to 30%. The hollow ratio less than 20% leads to increased weight of the fiber, whereas the ratio exceeding 30% leads to a fragile hollow form.

In the present invention, the MFI of each of polyethylene and polypropylene is controlled to cause the interface of the polyethylene portion of the conjugated fiber and the polypropylene portion of the fiber to be positioned at the center of the fiber, therefore resulting in exhibiting stretchability and crimp bulkiness.

Thusly prepared conjugated fiber is used to prepare the non-woven fabric. Through a carding process, the conjugated fiber is uniformly opened, arranged and bridged to form a web having a predetermined thickness, which is then passed through a hot air box heated to 130-140° C. at a line speed of 80-150 m/min, thus achieving thermal bonding of the web. Thereafter, the web is subjected to calendaring process, thereby forming a uniformly thick non-woven fabric. The temperature of the hot air box is adjusted in the range of 130-140° C., which is higher than the melting point of polyethylene. If the temperature of the hot air box is lower than 130° C., the thermal bonding is not sufficiently performed. While, if the temperature exceeds 140° C., the thermal bonding is excessively performed and the prepared non-woven fabric may be hardened. The faster the line speed, the higher the production efficiency. However, since the carding process and the thermal bonding process are carried out in the same line, the crimp is excessively opened at a line speed slower than 80 m/min, decreasing bulkiness of the non-woven fabric. At a line speed faster than 150 m/min, the crimp is thermally bonded in a less opened state, causing generation of a nap. Accordingly, the line speed is preferably in the range of 80-150 m/min.

A better understanding of the present invention may be obtained in light of the following examples which are set forth to illustrate, but are not to be construed to limit the present invention.

The following physical properties were measured as follows.

-   -   Melting point was measured by use of a differential scanning         calorimeter (purchased from Perkin-Elmer Co.);     -   Density was obtained by measuring a density 5 times using a         density gradient column and averaging the measured values;     -   Q (a ratio of weight average molecular weight to number average         molecular weight) was measured using p-xylene as a solvent by         gel permeation chromatography (using GPC purchased from Water         Co.);     -   MFI was obtained by measuring a melt viscosity 5 times at         190° C. in case of polyethylene and at 230° C. in case of         polypropylene using a melt flow indexer (purchased from         Davenport Co.) and 2160 g of a weight, and averaging the         measured values;     -   Weight was determined by weighing 9000 m of a unstretched fiber;     -   Bulkiness was calculated from (cm³/g)=(10.15×10.15         ×π×height)/sample weight, by placing 200 g of a prepared fiber         into a cylindrical container (diameter 203 mm×height 400 mm) and         applying a load of 100 g thereto, followed by determining a         height difference before and after the fiber was bulked; and

Stretchability was evaluated by thermally treating the prepared fiber at 120° C. for 3 minutes and comparing the change of the fiber length before and after thermal treatment.

EXAMPLE 1

As a primary component, polyethylene having a melting point of 130° C., a density of 0.9600 g/cm³, a value of Q of 5.0 and an MFI of 10 was used. In addition, as a secondary component, polypropylene having a melting point of 160° C., a density of 0.9100 g/cm³, a Q of 5.0 and an MFI of 16 was used. Polyethylene and polypropylene mixed at a weight ratio of 1:1 were added into a hollow and side-by-side type spinneret. As such, the spinneret had 2664 holes, and each hole was 0.5 mm across with a nozzle L/D of 6. A melt spinning process was performed at 260° C., to obtain an unstretched fiber, which was then stretched at a stretching ratio of 4:1 at a first stretching temperature of 88° C. and at a second stretching temperature of 93° C., and heated in a steam box. Then, pressure of a nib roller was controlled to have 12-20 crimps, and the fiber was cut to 51 mm long, to prepare a hollow and side-by-side type polyethylene/polypropylene conjugated fiber. The prepared conjugated fiber was subjected to carding process to make a web, which was passed through a hot air box heated to 140° C. at a line speed of 150 m/min, and subjected to calendaring process to have a uniform thickness to manufacture a non-woven fabric.

The physical properties of the conjugated fiber are shown in Table 1, below.

EXAMPLES 2-9

Conjugated fibers were prepared in the same manner as in the above example 1, except that MFI of each of polyethylene and polypropylene was changed. The physical properties of the prepared conjugated fiber are shown in Table 1, below.

COMPARATIVE EXAMPLES 1-3

Conjugated fibers were prepared in the same manner as in the above example 1, except that MFI of each of polyethylene and polypropylene was changed and a concentric sheath-core type spinneret was used. The physical properties of the conjugated fiber are shown in Table 1, below.

COMPARATIVE EXAMPLES 4-6

Conjugated fibers were prepared in the same manner as in the above example 1, except that MFI of each of polyethylene and polypropylene was changed and an eccentric sheath-core type spinneret was used. The physical properties of the conjugated fiber are shown in Table 1, below. TABLE 1 MFI Poly- Poly- Cross-Section Weight Bulkiness Stret. No. ethylene propylene Type (g) (cm³/g) (%) Ex. 1 10 10 Hollow, 1300 3000 20 Side-By-Side Type 2 10 16 Hollow, 1300 3400 22 Side-By-Side Type 3 10 25 Hollow, 1300 3400 19 Side-By-Side Type 4 18 10 Hollow, 1300 3000 18 Side-By-Side Type 5 18 16 Hollow, 1300 2500 16 Side-By-Side Type 6 18 25 Hollow, 1300 2800 17 Side-By-Side Type 7 25 10 Hollow, 1300 2400 17 Side-By-Side Type 8 25 16 Hollow, 1300 2200 17 Side-By-Side Type 9 25 25 Hollow, 1300 2200 16 Side-By-Side Type C. 1 18 10 Concentric Sheath- 1800 1000 0 Ex. Core Type 2 18 16 Concentric Sheath- 1800 1000 0 Core Type 3 18 25 Concentric Sheath- 1800 1000 0 Core Type 4 18 10 Eccentric Sheath-Core 1800 2800 15 Type 5 18 16 Eccentric Sheath-Core 1800 2400 13 Type 6 18 25 Eccentric Sheath-Core 1800 2700 15 Type

INDUSTRIAL APPLICABILITY

As described above, the hollow and side-by-side type polyethylene/polypropylene conjugated fiber of the present invention is advantageous in terms of excellent physical properties, such as light weight, high bulkiness and high stretchability. Using such a conjugated fiber, non-woven fabrics suitable for use in various applications are prepared.

The present invention has been described in an illustrative manner, and it is to be understood that the terminology used is intended to be in the nature of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. Therefore, it is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. 

1. A hollow and side-by-side type conjugated fiber comprising a primary component and a secondary component, said primary component being polyethylene with a melting point of 100-140° C., a density of 0.9500-0.9700 g/cm³, a molecular weight distribution index of 3.0-8.0 and a melt flow index of 10-25, and said secondary component being polypropylene with a melting point of 150-170° C., a density of 0.9000-0.9200 g/cm³, a molecular weight distribution index of 3.0-8.0 and a melt flow index of 10-25.
 2. The conjugated fiber as defined in claim 1, wherein a component ratio of polyethylene and polypropylene ranges from 3:7 to 7:3 based on weight.
 3. The conjugated fiber as defined in claim 1, wherein a hollow ratio of the hollow and side-by-side type polyethylene and polypropylene conjugated fiber ranges from 20 to 30%, and an interface of a polyethylene portion of the conjugated fiber and a polypropylene portion of the fiber is positioned at the center of the fiber.
 4. A method of preparing non-woven fabrics using the hollow and side-by-side type polyethylene/polypropylene conjugated fiber of any one of claims 1 to 3, comprising the steps of: performing a carding process of the polyethylene/polypropylene conjugated fiber, to make a web; passing the web through a hot air box heated to 130-140° C. at a line speed of 60-100 m/min; and performing calendaring process of the passed web. 